Laser q-switching

ABSTRACT

A transformer-driven electro-optic Q-switching arrangement reduces the high-voltage switching requirements with greatly simplified circuitry. Lasing efficiency is not significantly reduced even when the transformer rise time is twice as long as the laser pulse build-up time.

United States Patent [191 Hook et a1.

[451 Jan. 1,1974

[ LASER Q-SWITCHING [75] Inventors: William R. Hook, Los Angeles;

Ronald P. Hilberg, Redondo Beach, both of Calif.

[73] Assignee: TRW Inc., Redondo Beach, Calif.

22 Filed: June 19, 1972 [21] Appl. No.: 264,153

[52] US. Cl. 331/945 [51] Int. Cl. H015 3/12 [58] Field of Search331/945; 330/43;

[56] References Cited UNITED STATES PATENTS 3,521,069 7/1970 De Maria eta1 331/945 12/1971 Wuerker et a1 331/945 9/1972 Hook et a1. 331/945Primary Examiner-William L. Sikes Attorney-Daniel T. Anderson et al.

[ 5 7 ABSTRACT A transformer-driven electro-optic Q-switchingarrangement reduces the high-voltage switching requirements with greatlysimplified circuitry. Lasing efficiency is not significantly reducedeven when the transformer rise time is twice as long as the laser pulsebuild-up time.

5 Claims, 7 Drawing Figures 36 Vcf 1 Pygmgnm mm I 200 uuf GKV FLASH LAMPFIRES F LASER Q SWlTCHES V a. L l

-IOO us A! V ZV, /82

POWER SUPPLY (I KV) LASER Q-SWITCHING BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to Q-switching of lasers,and more particularly to a simplified circuit with reducedvoltage-switching requirements for modulating the electric field appliedto an electro-optic modulator, such as a Pockels cell, to therebymodulate the Q of the laser cavity.

2. Description of the Prior Art In the operation of an electro-opticallyQ-switched laser, it is normally necessary to electrically switch a highvoltage. The voltage to be switched usually ranges between 3 kV and 7kV, depending on the crystal and the wavelength. A lithium niobatePockels cell, 1 by l by 1 cm in size, which is'used to Q-switch a Nd:YAG laser, requires about 6 kV, for instance. A cold cathode gas tube,such as the EG&G type KN- 6 Krytron tube, is typically employed to shorta dc high voltage to ground. The Krytron tube is an attractive device,being of reasonable size and having a very fast switching time.Nevertheless, it is a gas tube with a limited lifetime and it doesrequire a fairly elaborate package in order to operate in extremeenvironments at such high voltages. It is clear that a circuit havingfewer components, operating at much lower voltages, and employing asolidstate switch rather than a gas tube would be desirable for bothreliability and economy. One possible method is to reduce the requiredswitching voltage by employing a much longer niobate crystal. Such acrystal is a good deal more expensive than a tube, however, thuspartially defeating the original purpose. A more attractive method is totake advantage of the fact that the Q- switching process is peculiarlyinsensitive to the actual loss function, and that maximum Q-switchingefficiency can be obtained as long as the loss in the cavity is lowduring the short interval of time that the output pulse is actuallycoming out of the laser. The loss during the relatively long build-upinterval does not affect the laser efficiency since there is negligibleexcited population depletion.

In a previous paper written by the inventorsherein, entitled, TransientElasto-Optic Effects and Q- Switching Performance in Lithium Niobate andKD*P Pockels Cells," published in Applied Optics, Vol. 9, pg. 1939, Aug.1970, it was reported that a lithium niobate modulator exhibits anoscillatory time dependent loss function that is caused by a largepiezo-electric effect in the modulator. It was further shown that such atime varying loss function can be altered, by'applying an appropriatebias voltage to the modulator, to reduce the Q-switching loss to zero ata time coincident with the occurrence of the laser pulse.

It is therefore a principal object of this invention to provide asimplified circuit having reduced switching speed and voltagerequirements for applying to an electro-optic modulator a voltagewaveform that generates a Q-switching loss function giving zero losswhen the laser pulse appears.

SUMMARY OF THE INVENTIO A step-up pulse transformer has its primaryconnected in a low voltage dc switching circuit. An electroopticmodulator in series with the secondary has a dc voltage applied thatresults in a low Q or high loss condition when the modulator is placedin a laser cavity.

When the primary circuit is switched to initiate a transient oscillatorycurrent, a transient alternating voltage pulse is induced in thesecondary in a sense that opposes the dc voltage across the modulatorand with a peak voltage swing that is at least about twice the amplitudeof the dc voltage. As a result of the modulating voltage applied, theO-switching loss function exhibited by the modulator is such that zeroQ-switching loss is experienced when the laser pulse occurs.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of aQ-switched laser in which the circuit of the invention finds use;

FIG. 2 is a circuit diagram of one embodiment of the invention showingan electro-optic modulator driven by a transformer;

FIG. 3 is a graph of waveforms useful in explaining the operation of thecircuit of FIG. 2;

FIG. 4 is a circuit diagram showing another embodiment of the invention;

FIG. 5 is a graph of waveforms useful in explaining the operation of thecircuit of FIG. 4;

FIG. 6 is a circuit diagram showing yet another embodiment of theinvention; and

FIG. 7 is a graph of waveforms useful in explaining the operation of thecircuit of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a schematic diagram of a Q-switched laser, including a laserrod 10, of ruby, neodymium in glass, calcium tungstate, or neodymium inyttrium aluminum garnet (NdzYAG), for example. Next to the laser rod 10there is mounted a pumping means, such as a xenon flash lamp 12, whichis electrically excited by voltage from a flash lamp driver 14.

The laser rod 10 is mounted in an optical cavity between two mirrors,namely, a front mirror 16 and a rear mirror 18. The front mirror 16 mayhave 40 to 50 percent optical reflectivity to transmit the output laserbeam, and the rear mirror 18 may be substantially totally reflecting. I

A light polarizer 20 is mounted between one end of the laser rod 10 andthe rear mirror 18, and an electrooptic modulator 22 is positionedbetween the light polarizer 20'and the rear mirror 18. The electro-opticmodulator 22 is preferably a Pockels cell made from lithium niobate andprovided with a pair of electrodes 24 and 26 on opposing surfacesthereof. So mounted,

together, the light polarizer 20 and the electro-optic modulator 22comprise a Q switch.

The electro-optic modulator 22 is energized by applying voltage to thetwo electrodes 24 and 26 from a Q-switch circuit 28. As shown, theQ-switch circuit 28 may receive an appropriately delayed signal from theflash lamp driver 14 to alter the output from the Q- switch circuit 28and hence the voltage applied to the electro-optic modulator 22.

As described thus far, the Q-switched laser is substantially the same instructure and operation as the one disclosed in U. S. Pat. No. 3,497,828issued Feb. 24, 1970 to C. L. Telk et al. In operation, the flash lamp12 receives a pulse of current from the flash lamp driver 14 and emits apulse of pumping radiation of the appropriate wavelength to excitemolecules in the laser rod 10. Light is emitted from the laser rod 10with different planes of polarization, but the light polarizer 20 isadapted to transmit light of one plane of polarization only. Forexample, if the light polarizer 20 is adapted to pass verticallypolarized light only, then only the light that is emitted from the laserrod with a polarization lying in a vertical plane will pass through thelight polarizer and impinge on the electro-optic modulator 22. The fastand slow axes of the electrooptic modulator 22 are oriented at 45relative to the direction of polarization of the laser light impingingthereon.

At the same instant of time when the flash lamp driver 14 is actuated,or prior thereto, the electrodes 24 and 26 of the electro-opticmodulator 22 receive a direct current voltage from the Q-switch circuit28, thereby impressing an electric field on the modulator 22. Thevertically polarized light exiting from the light polarizer 20 andentering the electro-optic modulator 22 is subjected to the electricfield during its traversal of the electro-optic modulator 22. The effectof the electric field on the polarized light is such that one of theorthogonal light radiation components aligned with the axes of theelectro-optic modulator 22 experiences a phase shift retardation of 90or one-fourth wavelength relative to the other component.

Upon reflection of the light from the rear mirror 18 and its traversalof the electro-optic modulator 22 in the opposite direction, the samecomponent of light is retarded an additional 90 or one-fourthwavelength, giving a combined phase shift of 180 or one-half wavelengthbetween the two orthogonal light radiation components. The vector sumof'these two light radiation components now results in a radiationpolarization that is rotated 90 from the original polarization vector.That is, the light energy that was vertically polarized before itentered the electro-optical modulator 22 is now horizontally polarizedafter it passes twice through the modulator 22, first in one directionand then in the opposite direction.

When the rotated or horizontally polarized light energy enters the lightpolarizer 20, it does not pass through the latter but rather, it isdeflected off the laser beam axis into a lossy element 30 mounted on aside of the light polarizer 20, where the light is absorbed. Under thiscondition, the optical cavity of the laser is said to be in a low Q or ahigh loss condition. The radiant power entering the Q switch isrepresented in FIG. 1 by the arrow P and the radiant power deflected outof the cavity or lost is represented by the arrow P The ratio of P /P isdefined as the Q switched cavity loss.

Recapitulating, when the flash lamp 12 is excited by the flash lampdriver 14, it initiates the pumping action on the laser rod 10 to causea build-up of excited atoms therein, and during that time voltage isimpressed on the electro-optic modulator 22 to establish a low Qcondition in the laser cavity. Allof the laser radiation is therebyconfined within the laser cavity and, since no lasing. build-up canoccur because of the low Q condi tion, no laser output beam can issuefrom the front mirror 16.

After a predetermined period of time has elapsed to permit the excitedatoms in the laser rod to reach a state of high population inversion,the Q-switch circuit 28 operates to alter the voltage on theelectro-optic modulator 22, that is, to switch it to zero and therebyestablish in the cavity a low loss or high 0 condition. In accordancewith the invention, the Q-switch circuit 28 generates a voltage waveformwhich, when impressed on the electro-optic modulator 22, alters thetransmission characteristics of the Q switch in such a way that itreaches its maximum transparency at the very moment the laser outputpulse appears. In FIG. 1, the output radiant power is represented by thearrow P The interval between the time when the voltage on theelectro-optic modulator is switched and the time when the laser outputpulse appears is known as the laser build-up time and is acharacteristic that is fixed by the particular design of the laser. Fora solid state laser, the laser build-up time is typically in the rangeof 50 to 200 nanoseconds.

Reference is now made to FIG. 2 which shows a detailed schematic diagramof the Q-switch circuit 28. A trigger voltage e,, which may be derivedfrom the flash lamp driver 14 of FIG. 1, is applied through a seriesresistor 32 to the gate electrode of a silicon-controlled rectifier 34.The cathode of the rectifier 34 is grounded and the anode thereof isconnected through a dropping resistor 36 to a positive dc voltage supplyV The anode of the rectifier 34 is connected through a blockingcapacitor 38 to one side of the primary Winding ofa pulse transformer40. The other side of the primary winding and one side of the secondarywinding are connected to a common ground.

The high voltage side of the secondary winding of the transformer 40 isconnected through a blocking capacitor 42 to one electrode 26 of theelectro-optic modulator 22, such as a Pockels cell, the other electrode24 being grounded. The high voltage electrode 26 is also connected inseries with a dropping resistor 44 to a high positive dc voltage V,,.

The values given for the circuit components and voltages are exemplaryfor use with an Airtron one-fourth inch by 3 inch Nd:YAG laser rod and a1 cm by 1 cm by 1 cm lithium niobate Pockels cell modulator in a cavity18 inches long with a 40 percent output reflectivity.

The operation of the circuit of FIG. 2 will now be described with theaid of the graph of waveforms of FIG. 3, wherein waveform (a) is thetrigger voltage q; waveform (b) is the transformer secondary voltageV,,; wave-form (c) is the voltage V across the electrooptic modulator;waveform (d) is the power loss ratio P /P in the cavity; and waveform(e) is the laser output power P,,.

Prior to the application of the trigger voltage 2,, thesilicon-controlled rectifier 34 is OFF, thereby blocking current flowtherethrough in either direction. The capacitor 38 is charged to thepotential of the voltage supply V or 330 volts. The voltage across theelectrooptic modulator 22 is equal to the high positive dc voltage V,,or 4.5 kilovolts.

At a time t,, after pumping of the laser is initiated and when theexcited atoms in the laser rod have reached a state of high populationinversion, the trigger voltage pulse e, is applied to the gate electrodeof the silicon controlled rectifier 34, whereby the latter is switchedON to provide a discharge path for the capacitor 38. When the capacitor38 discharges through the primary winding of the transformer 40, thesudden change in current causes a transient oscillatory voltage pulse V,to appear in the secondary. The transformer secondary winding is phasedsuch that the voltage pulse V,, is a negative going pulse, and the turnsratio is such that the maximum amplitude of the voltage pulse V isapproximately twice the dc voltage V;,.

The transformer secondary voltage pulse V has a rise time of about 250nanoseconds when a type TR148A transformer made by EG&G is used in thecircuit of FIG. 2. As shown in waveform (b) of FIG. 3, the rise time isthe time it takes the leading edge of the transformer secondary voltagepulse V, to rise from of its peak value to 90 percent of its peak value.

The voltage V,,, across the electro-optic modulator 22 is shown inwaveform (c) to fall from a value of 4.5 kilovolts through zero to anegative value of about 4.5 kilovolts in a little over 250 nanoseconds.The peak power output P of the laser pulse is shown in waveform (e) asoccurring at a time t spaced from time t. by the laser build-up time TAt time the voltage across the electro-optic modulator 22 is a negativevalue approximately midway between zero and its maximum negative value.Despite the fact that the electro-optic modulator 22 has a substantialvoltage impressed on it when the output laser pulse appears, waveform(d) shows that the cavity power loss is at its minimum or zero value attime 1 when the output laser pulse reaches its maximum value. At thistime the Q of the cavity is maximum.

Performance tests conducted in conjunction with the circuit of theinvention show that the lasing efficiency closely approximates thatobtainable from prior art circuits employing a gas tube to discharge thevoltage on a capacitor. There is somewhat higher loss in lasingefficiency at higher power levels as the pulse build-up time becomesmuch shorter than the transformer rise time. For instance, in a laserQ-switch circuit employing a 250 nanosecond rise time transformer, theloss in lasing efficiency at a power output level of ISO-millijoules was4 percent as compared with an efficiency loss of 2 percent at the 100millijoules level. Tests further show that the optimum lasing efficiencycan be achieved by using a transformer with a shorter rise time.

Whatever loss in lasing efficiency that occurs is more than compensatedfor by the circuit simplification and higher reliability resulting fromreduced voltage switching requirements. For example, comparing thepresent circuit employing a pulse transformer and a silicon controlledrectifier switch with a prior art circuit employing a high voltage gastube switch, the trigger voltage required to switch the siliconcontrolled rectifier is only 10 volts as compared with 500 volts appliedto the grid of the gas tube. Furthermore, the voltage switched by thesilicon controlled rectifier is only 330 volts as compared with over6,000 volts switched by the gas tube. Thus, an important feature of thecircuit of the present invention is the ability to switch thetransformer pri- -mary with a low voltage and thereby generate a highvoltage pulse in the secondary to drive the electro-optic modulator withthe desired waveform.

FIG. 4 shows a modified circuit in which the separate high voltage dcsource is eliminated, and instead the high initial dc voltage applied tothe electro-optic modulator is generated by rectifying a portion of thetransformer secondary voltage. A voltage divider including resistors 50and 52 of equal resistance value are connected across the secondarywinding of the transformer 40. One electrode of the electro-opticmodulator 22 is connected to the high voltage side of the transformersecondary winding and the other electrode is connected to the anode of arectifier diode 54 and to one side of a high voltage capacitor 56. Theother side of the capacitor 56 is grounded, and the cathode of the diode54 is connected to the junction of the resistors 50 and 52.

With the application of several trigger pulses at intervals of 20 to 100milliseconds through a series resistor 58 to the gate electrode of thesilicon controlled rectifier 34, the high voltage capacitor 56 chargesup to a negative dc voltage equal to one-half the peak value of thetransformer secondary voltage pulse. The voltage across theelectro-optic modulator 22 is then a positive dc value equal to thecapacitor voltage. Thereafter the Q-switching circuit may be operated to0 switch the laser in a manner similar to that described above inconnection with the circuit of FIG. 2. FIG. 5 shows waveforms of thetransformer secondary voltage V, and the capacitor voltage V Inpractice, it has been found that the high voltage capacitor 56discharges within several seconds after removal of the trigger voltagepulses e the discharge being effected through the back resistance of therectifier diode 54. Accordingly, the mere removal of the trigger pulsesfrom the silicon controlled rectifier 34 in the circuit of FIG. 4provides a convenient means for removing the high voltage from theelectro-optic modulator 22 whenever it is found necessary to conductcheckout and troubleshooting operations.

FIG. 6 shows an embodiment of the invention wherein a common circuit isused to fire the flash-lamp 12 that is used to pump the laser rod 10, aswell as to Q-switch the electro-optic modulator 22. First and secondtime-spaced trigger pulses e and e are coupled to parallel connectedswitching circuits in the primary of a pulse transformer 60. Theswitching circuits include resistors 62a, 62b through which the triggerpulses e and e are coupled to gate electrodes of the silicon controlledrectifiers 64a, 64b respectively. Capacitors 66a, 66b and chargingresistors 68a, 68b complete the primary switching circuits.

The secondary winding is connected to two parallel branches. One branchcontains the electro-optic modulator 22 in series with a capacitor 70which is initially charged to a high negative voltage through a chargingresistor 72. I

The other branch connected to the secondary winding includes a resistor74 connected to a trigger electrode 76 of the flash lamp 12. The triggerelectrode 76 may comprise the usual laser head in which both the laserrod 10 and the flash lamp 12 are housed adjacent to each other. Theelectrodes of the flash lamp 12 are connected to a discharge circuitincluding a capacitor 78 and inductor 80. The charging circuit for thecapacitor 78 includes a dc power supply 82 and charging resistor 84. Adiscussion of the process of flash lamp triggering is contained in apaper by William R. Hook, R. H. Dishington, and Ronald P. Hilberg,entitled Xenon Flashlamp Triggering for Laser Applications, published inthe IEEE Transactions on Electron Devices, Vol. EO-l9, No. 3, Mar. 1972,page 308.

The operation of the circuit of FIG. 6 will now be described with theaid of the waveform graph of FIG. 7. The first trigger pulse e coupledto the gate electrode of the first silicon controlled rectifier 64aswitches the latter ON, causing the first capacitor 66a to dischargecurrent through the primary of the transformer 60. The first highvoltage pulse V produced in the secondary drives the electro-opticmodulator 22 with a voltage V,,,, but since no pumping of the laser rodhas yet begun the drive on the electro-optic modulator 22 has no effecton the laser.

The first high voltage pulse V is also applied to the starting electrode76 of the flash lamp 12, causing the latter to partially ionize andprovide a low resistance path for the capacitor 78, previously chargedto the voltage of the power supply 82, to discharge. The resulting largecurrent causes the flash lamp 12 to emit the desired pumping radiationfor the laser rod.

After a sufficient pumping time has elapsed to permit the laser rod toreach the required state of high population inversion, the secondtrigger pulse e is applied to the gate electrode of the second siliconcontrolled rectifier 64b. By this time, the first trigger pulse e hasterminated. Upon application of the second trigger pulse e a secondvoltage pulse V,, is generated in the secondary of the transformer 60.The second voltage pulse V appears about 100 microseconds after thefirst one and is slightly reduced in amplitude due to the loading effectof the conducting flash lamp 12. The resistor 74 reduces the loadingeffect. The second voltage pulse V,, drives the electro-optic modulator22 in a manner similar to that described above in connection with thecircuits of FIGS. 2 and 4. About milliseconds later, the next twotrigger pulses e and e are applied to generate two new highvoltagepulses V,, to again drive the flash lamp l2 and the electro-opticmodulator 22 in succession.

What is claimed is:

l. A Q-switched laser, comprising:

a. a laser medium, a light polarizer, and an electrooptic modulatorarranged in an optical cavity;

b. means for applying an initial direct current voltage across saidelectro-optic modulator to establish a low Q condition in said opticalcavity; and

c. means for varying the voltage across said electrooptic modulator tothereby bring about a high Q condition in said optical cavity, said lastmentioned means including 1. a step-up pulse transformer having primaryand secondary windings:

2. a low voltage switching means in circuit with said primary windingfor initiating a transient current therein to thereby produce in saidsecondary winding an alternating voltage pulse having a peak voltageswing of greater amplitude than said initial direct current voltageacross said electrooptic modulator; and

3. means for coupling said alternating voltage pulse to saidelectro-optic modulator in a sense opposing said initial direct currentvoltage;

4. the amplitude and rise time of said alternating voltage pulse beingsuch as to cause said electrooptic modulator to exhibit a Q-switchingloss function, which is defined as the ratio of the radiant powerdeflected out of said optical cavity to the radiant power entering saidlight polarizer, that is substantially zero when the output laser pulseoccurs.

2. The invention according to claim 1, wherein said low voltageswitching means includes a capacitor charging circuit in series withsaid primary winding, and a solid state switch responsive to a triggerpulse for providing a discharge path for said capacitor through saidprimary winding.

3. A Q-switched laser, comprising:

a. a laser medium, a light polarizer, and an electrooptic modulatorarranged in an optical cavity;

b. a flash lamp optically coupled to said laser medium and having atrigger electrode;

c. means for applying an initial direct current voltage across saidelectro-optic modulator to establish a low Q condition in said opticalcavity;

d. a step-up pulse transformer having primary and secondary windings;

e. switch means in circuit with said primary winding and responsive tofirst and second time spaced trigger pulses for initiating correspondingtransient currents therein to produce in said secondary windingcorresponding first and second time spaced alternating voltage pulses ofgreater amplitude than said initial direct current voltage across saidelectro-optic modulator;

f. means for coupling said first alternating voltage pulse to saidtrigger electrode to excite said flash lamp for initiating pumping ofsaid laser medium; and

g. means for coupling said second alternating voltage pulse to saidelectro-optic modulator in a sense opposing said initial direct currentvoltage to thereby increase the Q of said cavity to a substantiallyhigher value;

h. the amplitude and rise time of said alternating voltage pulse beingsuch as to cause said electro-optic modulator to exhibit a Qswitchingloss function, which is defined as the ratio of the radiant powerdeflected out of said optical cavity to the radiant power entering saidlight polarizer, that is substantially zero when the output laser pulseoccurs.

4. The invention according to claim 3, wherein said switch meanscomprises first and second parallel connected switching circuits, eachincluding a capacitor charging circuit in series with said parimarywinding, and a solid state switch responsive to a corresponding one ofsaid trigger pulses for providing a discharge path for said capacitorthrough' said primary winding.

5, The invention according to claim 3, and further including:

1. means connecting one side of said electro-optic modulator to the highside of said transformer secondary winding;

2. a negative direct current supply connected to the other side of saidelectro-optic modulator; and

3. an isolating resistor connected btween the high side of saidtransformer secondary winding and the trigger electrode of said flashlamp.

1. A Q-switched laser, comprising: a. a laser medium, a light polarizer, and an electro-optic modulator arrangEd in an optical cavity; b. means for applying an initial direct current voltage across said electro-optic modulator to establish a low Q condition in said optical cavity; and c. means for varying the voltage across said electro-optic modulator to thereby bring about a high Q condition in said optical cavity, said last mentioned means including
 1. a step-up pulse transformer having primary and secondary windings:
 2. a low voltage switching means in circuit with said primary winding for initiating a transient current therein to thereby produce in said secondary winding an alternating voltage pulse having a peak voltage swing of greater amplitude than said initial direct current voltage across said electro-optic modulator; and
 3. means for coupling said alternating voltage pulse to said electro-optic modulator in a sense opposing said initial direct current voltage;
 4. the amplitude and rise time of said alternating voltage pulse being such as to cause said electro-optic modulator to exhibit a Q-switching loss function, which is defined as the ratio of the radiant power deflected out of said optical cavity to the radiant power entering said light polarizer, that is substantially zero when the output laser pulse occurs.
 2. a low voltage switching means in circuit with said primary winding for initiating a transient current therein to thereby produce in said secondary winding an alternating voltage pulse having a peak voltage swing of greater amplitude than said initial direct current voltage across said electro-optic modulator; and
 2. The invention according to claim 1, wherein said low voltage switching means includes a capacitor charging circuit in series with said primary winding, and a solid state switch responsive to a trigger pulse for providing a discharge path for said capacitor through said primary winding.
 2. a negative direct current supply connected to the other side of said electro-optic modulator; and
 3. an isolating resistor connected btween the high side of said transformer secondary winding and the trigger electrode of said flash lamp.
 3. A Q-switched laser, comprising: a. a laser medium, a light polarizer, and an electro-optic modulator arranged in an optical cavity; b. a flash lamp optically coupled to said laser medium and having a trigger electrode; c. means for applying an initial direct current voltage across said electro-optic modulator to establish a low Q condition in said optical cavity; d. a step-up pulse transformer having primary and secondary windings; e. switch means in circuit with said primary winding and responsive to first and second time spaced trigger pulses for initiating corresponding transient currents therein to produce in said secondary winding corresponding first and second time spaced alternating voltage pulses of greater amplitude than said initial direct current voltage across said electro-optic modulator; f. means for coupling said first alternating voltage pulse to said trigger electrode to excite said flash lamp for initiating pumping of said laser medium; and g. means for coupling said second alternating voltage pulse to said electro-optic modulator in a sense opposing said initial direct current voltage to thereby increase the Q of said cavity to a substantially higher value; h. the amplitude and rise time of said alternating voltage pulse being such as to cause said electro-optic modulator to exhibit a Q-switching loss function, which is defined as the ratio of the radiant power deflected out of said optical cavity to the radiant power entering said light polarizer, that is substantially zero when the output laser pulse occurs.
 3. means for coupling said alternating voltage pulse to said electro-optic modulator in a sense opposing said initial direct current voltage;
 4. the amplitude and rise time of said alternating voltage pulse being such as to cause said electro-optic modulator to exhibit a Q-switching loss function, which is defined as the ratio of the radiant power deflected out of said optical cavity to the radiant power entering said light polarizer, that is substantially zero when the output laser pulse occurs.
 4. The invention according to claim 3, wherein said switch means comprises first and second parallel connected switching circuits, each including a capacitor charging circuit in series with said parimary winding, and a solid state switch responsive to a corresponding one of said trigger pulses for providing a discharge path for said capacitor through said primary winding.
 5. The invention according to claim 3, and further including: 